Mercury Laser Altimeter (MLA)

NSSDC ID: 2004-030A-05Mission Name: MESSENGER

Description

The Mercury Laser Altimeter (MLA) is designed to measure the topography of the northern hemisphere of Mercury where the MESSENGER orbit is less than 1000 km above the surface. The primary scientific objectives for the instrument are to help characterize Mercury's geologic history and the size and state of the core. Altimetric mapping will be possible over nearly the entire northern hemisphere, yielding topographic maps and, in combination with other data, information on Mercury's global shape, spin axis, and libration. A total of roughly 1.5 Gb of data are expected over the nominal 1 year mission.

The MLA is based on the instruments flown on the Mars Global Surveyor and to be flown on ICESat. The MLA works by transmitting a laser beam to the surface, detecting the return reflection, and measuring the round trip time which, with accurate orbit determinations, gives the distance from the surface to the planet's center of mass. The laser is a Q-switched, diode-pumped Cr:Nd:YAG 1064 nm transmitter. The laser fires a 20 mJ pulse with a full-width, half-maximum duration of 5 ns. It is detected by an optical fiber to initiate the start of the timing process. The pulse passes through a beam expander and exits with a divergence of less than 50 microrad. The back-scattered laser echoes are collected by a receiver system consisting of four sapphire lenses. The returns then pass through a heat rejection filter and an optical bandpass filter to a hybrid avalanche photodiode. The transit tme is measured with an accuracy of 3.3 ns by the time interval unit, which corresponds to a resolution of 0.5 m. The laser pulse rate is 5 Hz. The spot diameter is 10-50 m and spot spacing is 100-300 m along track.